In general, in a plant, each kind of wide variety pipe is complicatedly installed. A thickness measurement technique by using an ultrasonic wave is well known as a technique of measuring thickness of these pipes. For example, a pipe inspection apparatus provided an ultrasonic wave probe with an outer surface of a pipe covered with a heat insulation material is disclosed in a Patent Document 1. In the pipe inspection apparatus, since an inspection device is always attached to the gap between the pipe and the heat insulation material, it is unnecessary to strip the heat insulation material from the pipe. Further, the pipe inspection apparatus can use for inspecting the pipe installed in the plant even in a driving (working) state of the plant.